dd15 air compressor safety valve quotation

hey guys i have a 2004 freightliner century st with the detroit 60 series 12.7 egr engine...i had the truck sitting in the yard for about a year and i would start it once a week or so,i was saving money for an inframe motor job because i had compression in the radiator...anyways on my way to the shop i was going too,to get an engine inframe done on it,the air compressor safety valve was popping like a fire cracker...i only drove the truck 20 miles to the shop and right before i got to the shop the air compressor safety valve was popping air out of it...the mechanic did the inframe engine job already..and now its time to figure out whats wrong with the air compressor safety valve popping off air like that...well we started already by changing the safety valve on top of the compressor but that didnt fix it...you guys know what it might be???from what this mechanic is saying is,it could be the air dryer filter or the compressor...